The present invention relates to a method and apparatus for shielding a sensor device. More specifically, the present invention relates to shielding a capacitive-type sensor.
Detection devices determine when a particular event occurs. Examples of detection devices include beam sensors such as those used in automatic doors. The beam sensor determines when a person has approached the door by detecting when the beam has been reflected or broken. Once the person is detected by the sensor, the door is automatically opened for the user. Other examples include label sensors. Label sensors detect labels as they pass by the sensor on a web. The detection of the label allows proper high-speed counting of labels as well as proper removal of the labels from the web.
For a detection device to properly function, it must be shielded from interference that may influence the detection device""s operation. In the case of capacitive type sensors, the sensor electrode must be isolated from undesired ground surfaces because the sensor is intended to measure the capacitance between a sensor electrode and ground electrode and undesired ground surfaces affect that capacitance value. Objects moving in proximity to the sensor and the undesired ground surfaces will change the capacitance between the sensor electrode and the ground surfaces which may result in a false sensing condition. To reduce the effects of the undesired ground surfaces, a shield is provided to surround the sensor electrode except on the side facing the ground electrode. However, the shield itself may act as an undesired ground surface that influences the signal applied to the sensor and may cause false sensing conditions to occur.
The present invention addresses these problems by driving the shield to a potential matching the potential of the sensor electrode. The driving signal is independently created which allows the potential of the shield to closely match the potential of the sensor electrode during periods when the sensor electrode""s potential is varying rapidly.
The invention is embodied in apparatuses and methods for providing a capacitive-type label sensor for sensing an object where the sensor has a reference electrode coupled to a first side of an opening and a sensor electrode coupled to a second side of the opening. The method includes electrically connecting the reference electrode to system ground and electrically connecting the sensor electrode to a first end of a first resistor and a first electronic driving circuit at a first connection where the first resistor is electrically coupled between the first connection and a supply voltage. The method also includes electrically connecting a shield electrode to a first end of a second resistor and a second electronic driving circuit at a second connection where the second resistor is electrically coupled between the second connection and the supply voltage. The method further includes determining the edge of the object passing through the opening by detecting a change in capacitance between the sensor electrode and the reference electrode.
The apparatus includes a reference plate coupled to the first side of the opening that is electrically connected to ground and includes a first electronic driving circuit for generating an output signal related to the measured capacitance within the opening. Also included in the apparatus is a sensor electrode coupled to the second side of the opening and electrically connected to the first electronic driving circuit at a first connection. A first resistor is included and is electrically connected between the first connection and a supply voltage. The apparatus further includes a second electronic driving circuit and includes a shield electrode coupled to the second side of the opening and electrically connected to the second electronic driving circuit at a second connection. A second resistor is included and is electrically connected between the second connection and the supply voltage. A low pass filter circuit is included and generates a capacitance signal from filtering the output signal.